The invention relates to a method of manufacuturing a semiconductor device, in which a p-type dopant is provided at a part of a surface of a p-type monocrystalline silicon substrate and is diffused into the substrate, an n-type epitaxial silicon layer is then provided on the surface, after which a p-type dopant is provided at a part of the surface of the epitaxial layer opposite said part of the substrate surface, wherein the p-dopants are diffused into the epitaxial layer from two sides by a thermal treatment to form diffused p-type regions which meet each other in said layer.
A method having these features is disclosed in Netherlands Patent Application No. 6700755, in which the formation of island-insulation regions is described. As a p-type dopant, boron is locally introduced into a silicon substrate so as to obtain a highly boron-dopant region.
Such a highly doped region in the substrate creates a problem in that epitaxial silicon layers which are deposited thereon may not have the desired composition due to autodoping with the boron of the doped region.